1. Field of the Invention
The present invention relates to an aqueous ink composition suitable for an ink-jet printer, aqueous writing utensils, various kinds of recorders and pen plotters, and more particularly to an aqueous ink composition capable of producing color images on a sheet of plain paper by the ink-jet printing method. In addition, the present invention relates to an ink-jet printing method for recording color images on a sheet of plain paper using the above-mentioned aqueous ink composition.
2. Discussion of Background
The ink-jet printer has been widely utilized recent years because of its advantages of low noise and low running cost, and a color ink-jet printer capable of producing color images on a sheet of plain paper has been on the market. With respect to the conventional ink-jet printers, however, all the requirements for the obtained color images, such as color reproduction, water-resistance, light-resistance, drying characteristics and image quality without blurring, and the reliability of ink-ejection from the printer cannot be satisfied. Particularly, when the color images are produced by using the color ink-jet printer, the deterioration in image quality takes place in an image portion of a secondary color such as red, green or blue obtained by superimposing the colored inks, even though the image quality of color images of yellow, magenta or cyan scarcely deteriorates.
Further, in the case where the ink deposited on a sheet of paper is dried without any specific image-fixing unit, it is necessary to improve the drying characteristics of the ink by increasing the penetrating property of the ink through the paper, as described in Japanese Laid-Open Patent Application 55-29546. Because of the increase in the penetrating property of ink through the paper, obtained images are easily blurred depending upon the kind of paper.
It is described in Japanese Patent Publication 60-23793 that not only the drying characteristics of ink can be improved, but also the deterioration of obtained images can be reduced when dialkylsulfosuccinic acid is used in the ink composition as a surface active agent. When such an ink composition is used for ink-jet printing, however, the diameter of a picture element of the obtained image remarkably varies depending on the kind of image-receiving sheet to be employed, and the decrease in image density of the printed images is conspicuous. In addition to the above, this kind of surface active agent is easily decomposed in an ink composition with a high alkaline strength, so that the activating effect of the surface active agent is impaired during the storage of the ink composition.
A quick-drying ink is disclosed in Japanese Patent Publication 58-6752, which comprises as a surface active agent an ethylene oxide adduct with an acetylene bonding. This quick-drying ink has the advantage that the penetrating property of the ink through a sheet of paper is improved owing to the ethylene oxide adduct contained in the ink composition, thereby reducing the blurring of ink on the paper. However, there is the problem that the drying rate of the ink cannot be increased according to the kind of dye contained in the ink composition. For example, when a direct dye such as DBK168 is used as the dye in the ink composition, the drying rate of the ink cannot be increased because of the hydrophobic interaction between the surface active agent and the dye.
An ink composition comprising a strongly basic material is disclosed in Japanese Laid-Open Patent Application 56-57862. The penetrating speed of this kind of ink composition through a so-called acidic paper, which has been sized by use of rosin, can be increased. However, even when the acidic paper is used, the penetrating speed of the ink through the paper is not increased when an image is obtained by superimposing two colored inks. In addition, the above-mentioned ink composition is not appropriate for the paper prepared by use of a sizing agent such as alkyl ketene dimer or alkenyl sulfosuccinic acid.
There is proposed a recording ink composition comprising a polyhydric alcohol derivative and pectin as disclosed in Japanese Laid-Open Patent Application 1-203483. In this ink composition, pectin is contained as a thickening agent to prevent the blurring of ink on the paper. When the ink composition is used for the ink-jet printer, however, ejection of the ink from the nozzles of the printer is not stable when the printing operation is resumed after intermission because pectin is a nonionic material having a hydroxyl group as a hydrophilic group.